fix: use safe arithmetic assignment to avoid exit-on-zero

With set -euo pipefail, ((COUNTER++)) returns 0 when COUNTER is 0,
causing the script to exit with error. Using COUNTER=$((COUNTER + 1))
is safe and always returns the new value.

This bug would cause the script to fail on the first skip, even though
skipping is valid behavior (e.g., backend plugins, missing metadata).

Reported-by: kadel
Ref: #2383 (comment)

Author: gustavolira

scripts/instrument-plugin.sh

@@ -56,22 +56,22 @@ while IFS= read -r PROD_IMAGE; do
 
   if [[ ! -f "$METADATA_FILE" ]]; then
     echo "  ⚠️  No metadata file found at $METADATA_FILE - skipping"
-    ((SKIPPED_COUNT++))
+    SKIPPED_COUNT=$((SKIPPED_COUNT + 1))
     continue
   fi
 
   # Check if this is a frontend plugin (only frontend plugins need instrumentation)
   PLUGIN_ROLE=$(yq -r '.spec.backstage.role // ""' "$METADATA_FILE")
   if [[ "$PLUGIN_ROLE" != "frontend-plugin" ]]; then
     echo "  Skipping $PLUGIN_ROLE (only frontend plugins need browser coverage)"
-    ((SKIPPED_COUNT++))
+    SKIPPED_COUNT=$((SKIPPED_COUNT + 1))
     continue
   fi
 
   # Pull production image first (needed to inspect labels)
   if ! podman pull "$PROD_IMAGE" 2>&1 | grep -v "WARNING: image platform"; then
     echo "  ❌ Failed to pull image - skipping"
-    ((SKIPPED_COUNT++))
+    SKIPPED_COUNT=$((SKIPPED_COUNT + 1))
     continue
   fi
 
@@ -109,7 +109,7 @@ while IFS= read -r PROD_IMAGE; do
 
   if [[ -z "$PLUGIN_PATH" ]]; then
     echo "  ⚠️  Could not determine plugin path - skipping"
-    ((SKIPPED_COUNT++))
+    SKIPPED_COUNT=$((SKIPPED_COUNT + 1))
     continue
   fi
 
@@ -121,7 +121,7 @@ while IFS= read -r PROD_IMAGE; do
     echo "  ❌ Failed to extract plugin bundle from container - skipping"
     podman rm "$CID" || true
     rm -rf "$WORK_DIR"
-    ((SKIPPED_COUNT++))
+    SKIPPED_COUNT=$((SKIPPED_COUNT + 1))
     continue
   fi
 
@@ -133,7 +133,7 @@ while IFS= read -r PROD_IMAGE; do
   if ! (cd "$WORK_DIR" && npx --yes nyc@18.0.0 instrument dist-original dist-instrumented --source-map); then
     echo "  ❌ Instrumentation failed - skipping"
     rm -rf "$WORK_DIR"
-    ((SKIPPED_COUNT++))
+    SKIPPED_COUNT=$((SKIPPED_COUNT + 1))
     continue
   fi
 
@@ -142,7 +142,7 @@ while IFS= read -r PROD_IMAGE; do
   if [[ "$JS_COUNT" -eq 0 ]]; then
     echo "  ❌ No __coverage__ found in instrumented files - skipping"
     rm -rf "$WORK_DIR"
-    ((SKIPPED_COUNT++))
+    SKIPPED_COUNT=$((SKIPPED_COUNT + 1))
     continue
   fi
   echo "  ✓ Instrumented $JS_COUNT JS files"
@@ -162,15 +162,15 @@ EOF
   if ! podman build -t "$COVERAGE_IMAGE" -f "$WORK_DIR/Containerfile" "$WORK_DIR"; then
     echo "  ❌ Failed to build coverage image - skipping"
     rm -rf "$WORK_DIR"
-    ((SKIPPED_COUNT++))
+    SKIPPED_COUNT=$((SKIPPED_COUNT + 1))
     continue
   fi
 
   # Push coverage image
   if ! podman push "$COVERAGE_IMAGE"; then
     echo "  ❌ Failed to push coverage image"
     rm -rf "$WORK_DIR"
-    ((SKIPPED_COUNT++))
+    SKIPPED_COUNT=$((SKIPPED_COUNT + 1))
     continue
   fi
 
@@ -180,7 +180,7 @@ EOF
   rm -rf "$WORK_DIR"
   echo ""
 
-  ((INSTRUMENTED_COUNT++))
+  INSTRUMENTED_COUNT=$((INSTRUMENTED_COUNT + 1))
 
 done